ARM: dts: lager: use demuxer for IIC2/I2C2

Create a separate bus for HDMI related I2C slaves.

Based on work by Wolfram Sang.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
[wsa: rebased, removed typo in comment, fixed aliases, switched to
named GPIOS, sort SCL pins first]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>